Traditionally, digitally controlled color printing capability is accomplished by one of two technologies. Both require independent ink supplies for each of the colors of ink provided. Ink is fed through channels to a nozzle set from which droplets of ink are selectively ejected. Typically, each technology requires separate ink delivery systems for each ink color used in printing.
Conventional “drop-on-demand” ink jet printers utilize a pressurization actuator to produce the ink jet droplet at orifices of a print head. Typically, one of two types of actuators are used including heat actuators and piezoelectric actuators. With heat actuators, a heater, placed at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a mechanical stress in the material causing an ink droplet to be expelled.
The second technology, commonly referred to as “continuous stream” or simply as “continuous” ink jet printing, uses a pressurized ink source which produces a continuous stream of ink droplets. Some continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no printing is desired, the ink droplets are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or discarded. When printing is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism.
U.S. Pat. No. 3,709,432, issued to Robertson on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced ink droplets through the use of transducers. The lengths of the filaments before they break up into ink droplets are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitudes resulting in long filaments. A flow of air is generated across the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow affects the trajectories of the filaments before they break up into droplets more than it affects the trajectories of the ink droplets themselves. By controlling the lengths of the filaments, the trajectories of the ink droplets can be controlled, or switched from one path to another. As such, some ink droplets may be directed into a catcher while allowing other ink droplets to be applied to a receiving member.
U.S. Pat. No. 6,079,821, issued to Chwalek et al. on Jun. 27, 2000, discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual ink droplets from a filament of working fluid and deflect thoses ink droplets. A print head includes a pressurized ink source and an asymmetric heater operable to form printed ink droplets and non-printed ink droplets. Printed ink droplets flow along a printed ink droplet path ultimately striking a print media, while non-printed ink droplets flow along a non-printed ink droplet path ultimately striking a catcher surface. Non-printed ink droplets are recycled or disposed of through an ink removal channel formed in the catcher. While this device is capable of high quality printing, it is limited to ink fluids which have a large viscosity change with temperature.
U.S. Pat. No. 6,554,410, which issued to Jeanmaire et al. on Apr. 29, 2003, and U.S. patent application Ser. No. 09/751,232, filed Dec. 28, 2000, disclose continuous-jet printing methods wherein nozzles with annular heaters are selectively actuated at a plurality of frequencies to create the stream of ink droplets having the plurality of volumes. A gas stream then separates droplets into printing and non-printing paths according to drop volume. Larger droplets are directed to a recording media, whereas smaller droplets are captured in a plenum and recycled.
For traditional color printing applications, three or four print heads are required (i.e., CMY or CMYK). The use of additional inks, for example, multiple concentrations of a colorant, can provide superior photographic reproduction as presented in U.S. Pat. No. 4,672,432 to Sakurada et al. in 1987. Six print heads were required, one for each of high density black, high density yellow, high density cyan, high density magenta, low density cyan and low density magenta. While this approach can improve the image quality for photographic printing, additional print heads significantly increase the cost of the apparatus.